Boatworks on Youtube is a really good resource on this. He also covers the different resins, and
gelcoat. You can pick up a west systems epoxy book from several places, and it is fairly detailed on repairs and epoxy. The prep work is the most important piece of the repair you are talking about, followed by using the correct amount of overlapping reinforcing layers of glass.
Epoxy resin is more expensive, but stronger and waterproof. It doesn’t smell. I would not use this if you intent to
gelcoat after, although gelcoat will stick to it, it’s just an
adhesive bond, so nothing I would use on a critical or structural part. Very easy to use. You get the metered hand pumps so your mix is 1
pump resin to 1
pump hardener, no chemistry, measuring, or precise volume control. Pick the hardener based on temperature and strength you need. Slow hardener is best for high temps but is weaker, fast hardener can be used at 30-40 degrees and is the strongest. Try to use fast hardener at 90 degrees and you have a hard smoking hockey
puck in 15 minutes. I have been using Slow hardener in the 90’s, making bigger batches and haven’t had any issues with time, hour or more easy. Epoxy almost sticks to everything. Never seen anything about old epoxy. It always seems to work. I have successfully used 2-3 year west systems epoxy, although for any structural stuff I’ll get fresh epoxy. Some epoxy’s like west systems cures with an amine layer that has to be scrubbed off with soap and water. Not a big deal but it’s an added step that isn’t always easy. Now a days I use amine free/ low amine epoxy’s from us composites and MAS. It’s a thinner laminating resin which makes it easier to soak into the glass. Thin resins are also good to thin out with alcohol or acetone to soak it into wood. Penetrates deeper, then coat with a coat of regular mix. I have saved some water damaged wood this way. Us composites is cheaper so I don’t mind using it to make a thick epoxy with fillers, but anytime you use a thin laminating resin for this you have to use a lot more filler powders to thicken it up. If I was specifically going to use a lot of thickened epoxy or fairing compound fillers, I would start off by getting a thicker epoxy. There are a lot of different brands of epoxy, they are more or less the same for what you are doing and any of them should work well for you. You will find more how to’s and 3rd party information and videos on west systems however, which I found is fairly easy to get.
Polyester resin – stinky stuff everyone things of when you talk about fiberglass. In-expensive, but not waterproof, not as strong as epoxy. If you’re doing a thick layup of glass you use this resin to keep cost reasonable. When you hear talk about
blisters under the waterline your talking about polyester resin. It’s common to add a couple coats of epoxy barrier coat over polyester resin so that its waterproof and prevents
blisters. Your working time is low, so small batches at a time. It won’t fully cure hard and will remain sticky if you don’t coat it with something that
seals it from air, usually wax additive or
mold release. You get resin without wax for laminating several layers in one setting, and you use a resin with wax as your final layer to cure everything, I still coat everything with
mold release just in case. There is chemistry, math and ratios you have to work with to mix this up, so not as easy to use as epoxy. Hardener is MEK and you adjust the amount based on total volume and temperature. Small batches you use a dropper bottle of MEX and count drops. Larger batches you measure out.
Mistakes may prevent your resin from curing at all, so it stays a semi hard sticky mess that you have to scrape and remove later to try again. Polyester resin doesn’t stick to everything. Gelcoat is a polyester resin, so you get a strong chemical bond between the fiberglass and gelcoat. Gelcoat is the reason its worth
learning to use polyester. Has a decent shelf life. I have been told ~ 1 yr, but that may have been just gelcoat. Have seen old polyester used successfully.
Vinylester resin. I haven’t used this, kind of a mix of the strengths of the other two resins. Mixing is by volume chemistry like polyester. It’s stronger than polyester, and waterproof. Cheaper than epoxy, not as strong. It is compatible with gelcoat or there is a vinylester version of gelcoat (not 100% on this). The big negative with this is it has a short shelf life of a couple months I believe. One of the boatworks videos compares the t=different resins and they couldn’t test the vinylester because it was passed its shelf date. So you buy this just before using it.
Specialty resins – there are specialty resins, or hybrid resins made for specific purposes. You don’t see these as often.
Fuel safe resins to coat your
fuel tanks, potable water safe resins, 2 part resin foams etc.
Gelcoat versus
paint – if you are going to paint than just use epoxy. It’s more expensive but more user friendly and easy to mix. You could try to
experiment with gelcoat over epoxy, not my recommendation for a first time user. If you are successful with polyester resin than gelcoat is not much different, almost like a thick paint. However it is really hard and a pain to sand. It has to be sanded with finer and finer sandpaper and then polish it. It was so easy and looks good initially when you put it on that the first time I patched a bigger spot I thought later I may spray my whole hull to renew it. Weeks of sanding and polishing have completely canceled that idea and I have decided if I need to do larger gelcoat patches ill just paint since it looks good and is less work. When it’s done right it looks good and is durable, but Pure misery sanding gelcoat, like doing millions of arm circles over your
head. There can be more chemical mixing oops with gelcoat just like polyester resin. I learned some new cuss words watching someone that had to remove gelcoat that didn’t cure from poor mix.
Regardless of the resin don’t buy the premade thickened stuff. I found it’s easier to mix your own and faster to use, if you get the powders and do it yourself. This lets you mix exactly what you need. Polyester kitty hair is often too stringy and can be a pain for more detailed work. But you can mix up something similar with short strands that works better in your application. I used to buy the premixed west system thickened epoxy in the caulking tube. Its $40 a tube at west
marine. You get two of the special mixing nozzles with that. Otherwise you buy additional nozzles in a 2 pack. You don’t always need that much in one setting and it goes to waste. I remember always going out of my way to make sure the two sides didn’t contaminate each other and harden. When you have the powders and additives you can mix the stuff to the consistency you need in the amount you need. Very easy. I got a 5QT bucket of cabosil/silica for $8, the west systems equivalent is $20-$30. You can also use a easy to sand fairing compound additive/powder to make a fairing compound that chemically bonds to the layer before, rather than the expensive purpose made fairing compounds.
I use mainly epoxy. It’s more expensive but I can buy in larger quantities and keep leftover in the lazarette for the next time I need epoxy. I have a Fiberglass boat, but the furniture and bulkheads are
teak marine (maybe) ply. I have found I need to treat more wood with several coats of thinned epoxy as the boat gets older. Polyester you need a full mask due to the vapers, very smelly, it will stink out your boat for a while after it cures. So epoxy avoids that in close confines, garages ect. The working time and ease of use makes it my go to stuff. Currently using US Composites 635 Thin Epoxy Resin System as it seems to be the best deal I found cost wise and I found I like it a lot because it’s easy to wet out. I call and order and they ship it in a few days without issue. I liked MAS a lot too. Don’t buy your resins from west marine, its 3 times as much there as you can order it including
shipping. Also don’t buy the west systems fillers, the fillers are the same and you can get the same stuff elsewhere in larger quantities for cheaper. The exception is some of west systems specialty fillers like barrier coat, graphite maybe, metallic powders ect.